Killer fungus goes airborne

A fungus that is devastating amphibian populations around the world is a relatively new disease that is spreading rapidly, rather than an old disease that has recently become more virulent, according to research on frogs in California's Sierra Nevada.

Worse, not only is the fungus being spread by infected water, it may also be transmitted in the form of spores carried on the wind or birds' feathers, for example, a genetic analysis of the Californian frogs suggests. This would help to explain outbreaks of the disease, called chytridiomycosis, in remote, inaccessible habitats like the Sierra Nevada lakes.

Knowing the disease may spread in this way will also bolster efforts to contain it. "This gives us a heads up that just controlling the amphibian trade for food and aquaria may not be enough," says Jess Morgan of the Animal Research Institute in Moorooka, Queensland, Australia, who led the work.

Fungus evolves

Morgan and her colleagues studied fungus samples taken from mountain yellow-tailed frogs from lakes around the Sierra Nevada. They found that the samples were genetically very similar, and also very similar to samples from other parts of the world, suggesting the fungus has spread globally only quite recently.

However, the team did find slight genetic differences in samples from different lakes. Researchers had thought that the fungus reproduces only asexually. "But the genetic evidence suggests the DNA is recombining in these separate sites - the fungus is starting to evolve," says Morgan.

Recombination of DNA means sexual reproduction and, based on the life cycles of related fungi, this is likely to involve the creation of fungal spores, Morgan says.

So far the team hasn't found any B. dendrobatidis spores, either in the wild or the lab. But for some related fungi, sexual reproduction, and hence spore production, is triggered only in certain circumstances such as when a water source dries up. The team plans more work to try to find a trigger for this species.

B. dendrobatidis was formally identified as a frog-killer in 1998, though it has been found on museum specimens dating back to the 1930s. It affects frogs on every continent they are found and is considered one of the three major reasons - along with habitat destruction and trade - for the decline in global amphibian populations since the early 1980s.

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